THURSDAY, May 12, 2016 (HealthDay News) -- Repeated blows to the head can cause near vision to blur slightly, even if the individual impacts aren't strong enough to cause a full-fledged concussion, a new study says.

During a regular football season, about two dozen college players developed a vision problem known as "near point of convergence," even though none suffered a concussion, according to the report.

Near point of convergence occurs when the muscles that control the eyes -- the ocular-motor system -- are unable to perfectly align both eyes to focus on close-up objects, said senior researcher Keisuke Kawata, a Ph.D. student at Temple University in Philadelphia.

As a result, words could appear blurry or doubled, Kawata said.

In the case of these players, the vision problem was so mild that they didn't notice it, Kawata said, and it didn't appear to affect them on the football field or in the classroom.

But vision tests showed the condition persisted throughout the football season, "because they were practicing almost every day," Kawata said.

The problem cleared up on its own within three weeks after the season, when players were no longer sustaining regular impacts during play, he said.

"This tells us that the ocular-motor system is very slow to recover, and it is vulnerable to even mild head impacts," Kawata said.

The findings raise concerns about potential long-term damage in athletes who play year-round, and never give their ocular-motor systems a chance to recover from continuous blows to the head, he said.

"You are sustaining a considerable amount of head impact," Kawata said. "It's not as strong as concussive blows, but if you sustain cumulative head impact for a long period of time, who knows 20 or 30 years later if that's going to affect anything?"

It's vital to understand the effects of repetitive subconcussive impacts because they occur more often than concussions. College football players are reported to sustain as many as 1,350 subconcussive head impacts a season, the researchers said in background notes.

People who suffer full-fledged concussions often complain of symptoms related to near point of convergence, said Dr. Andrew Lee, chair of ophthalmology at Houston Methodist Hospital in Texas.

"If you can't converge properly, you'll get a double image and things will be blurry," said Lee, who wrote an editorial accompanying the study.

The new report follows a previous study in which Kawata and his colleagues asked soccer players to perform 10 "headers," in which they used their heads to stop a soccer ball shot out of a pitching machine.

The researchers found the players' near point of convergence "was pretty noticeably worse, and this phenomenon was not recovered after one day of resting, which means it's a very slow recovery even after a mild head impact," Kawata said.

To take their study further, the research team recruited 29 Temple University football players. They fitted them with a "smart" mouthguard that tracked the number of head impacts received during play and the magnitude of head acceleration during those impacts.

The researchers then followed the players through five preseason practices, sorting them into two groups -- one that received higher-impact blows to the head during play, and those who received lower-impact blows. None of the blows was strong enough to cause a concussion.

These subconcussive head impacts didn't cause any symptoms noticeable to players, regardless of how often and how hard they were hit, researchers found.

However, vision tests found the higher-impact group suffered from compromised near point of convergence. Those vision problems remained three weeks later when tested at the end of training camp.

But after three weeks' rest postseason, their eyes returned to normal, according to follow-up vision exams.

The findings were reported May 12
in the journal JAMA Ophthalmology.

Although more research involving a larger group is needed, Lee said this pilot study shows a vision test could be useful in judging the severity of a brain injury, either on the sidelines or following a traumatic accident.

"It could be used possibly as a screening device and a diagnostic device," Lee said. "The convergence problem might be the tip of the iceberg, in terms of more significant neurocognitive deficits."